As is well known, in a semiconductor memory cell, data is stored by programming the cell to have a desired threshold voltage. Simple memory cells store one of two states, a logical one or a logical zero, in which case the cell is programmed to either turn on or not turn on, respectively, when read conditions are established, thereby allowing the read operation to determine if a logical one or a logical zero has been stored in the memory cell. More sophisticated semiconductor memory cells allow the storage of one of a plurality of memory states greater than two,--by providing the ability to store a variety of threshold voltages in the memory cell, each threshold voltage being associated with one of a plurality greater than two logical states. Such multi-state memory cells and arrays are described, for example in U.S. Pat. Nos. 5,043,940 and 5,434,825 issued on inventions of Dr. Eliyahous Harari.
In order to fully exploit the concept of high density multi-state memory devices, the memory states must be packed as closely together as possible, with minimal threshold separation for margin/discrimination overhead. Factors which dictate this overhead are noise, drift (particularly random as opposed to common mode), sensing speed (deltaT=C*delta V/I), and safety margin guard bands, as well as precision and stability of reference sources/sense circuits. This overhead must be added to the memory state width associated with precision of writing the memory cells (again with respect to the reference sources). With a closed loop write, in which a write is performed followed by a verify operation and in which cells which fail the verify operation are rewritten, the relative precision of memory cell to reference source can be made arbitrarily high by expending more time in writing. State packing will then be dictated more by how precise and stable the various storage sense points can be separated from one another, a property of both memory state stability and how reference points/elements are established.